Warpage is a condition where a U-shaped (or upside down U) bend is formed in a semiconductor package during the cooling down period immediately after a semiconductor package has been encapsulated with resin and been cured. Although this problem can occur with all types of semiconductor packages, it is most severe with the thinner packages such as thin quad flat packages (TQFP) used commonly in smart cards or in other applications where thinness is a high priority. Although the warpage may create functional problems of various kind, the most noticeable one is the misalignment of the leads caused by the shift in the angle at which the leads protrude out from the package. This misalignment can prevent the TQFP from mating properly with sockets or circuit boards.
FIG. 1 is an illustration of a TQFP with a warpage problem. The severity of the warpage is commonly quantified by measuring the distance marked "W" in FIG. 1. Currently in the industry, it is common to require a warpage specification of less than 50 microns. The industry so far has faced extreme difficulties in finding a high-yield process for producing packages which meet the required specification. By some estimates, the yield may be as low 20 to 30 percent, which is considered very low by the general semiconductor packaging standards. Hence it is not uncommon to find batches where a relatively large number of TQFP packages have warpage value of 70 to 90 microns. In light of this problem and the industry's failure to solve it, some industry users of TQFPs are forced to accept packages with warpage greater than 50 microns.
In an attempt to rectify the warpage problem, a number of solutions have been proposed and implemented, but without total success. One solution was to place the TQFP in a clamping device, as shown in FIG. 2, during the cooling period of the TQFP immediately after the resin has been cured. The clamping device has an upper plate 5 and bottom plate 7 both having a flat planar surface which makes contact with the TQFP 8. In an attempt to prevent the warpage from forming, a relatively small force (e.g., 400 N or 0.015 N/mm.sup.2) is exerted onto the upper plate before the warpage has had time to form. The clamp is left in the clamped position until the package has sufficiently cooled to near room temperature.
Some of the semiconductor packing tools currently available are equipped with this clamping device in the cooling station of the packaging process. This clamp has helped to reduce the severity of the warpage to some degree, but still not enough to significantly increase the yield to acceptable level. Hence, many packages are still above the required specification of 50 microns and so there still exists much room for improvement.
To enhance the effect of the clamping process, it was proposed that the TQFP be placed in a negative clamp, such as the one shown in FIG. 3, during the cooling period. The TQFP 30 in FIG. 3 is a package which has the tendency to warp downward, i.e., warpage having the shape that of an upside down "U". To additionally compensate for this type of warpage, a cavity 35 is provided, in this case, on the bottom plate 40. Again relatively small force is applied on the upper plate 45 until the package bends slightly away from the direction of the warpage in attempt to compensate for the package's tendency to warp in the opposite direction. The package is then left to cool to near room temperature.
Like its predecessor, the negative clamping also did not provide completely satisfactory results as many TQFPs still experienced warpage problems beyond the required specification. Moreover, because of the possibility of damaging the packages due to possible over-compensation, the negative clamping is not accepted as a feasible solution and hence is not implemented.
At least a few other attempts have been made to solve the warpage problem. But to the best of the inventor's knowledge, currently, there exists no effective method and apparatus for eliminating or even significantly reducing the warpage in TQFPs, and certainly no effective method exists to which is simple to use and implement. In light of the seriousness of the warpage problem, and the industry's inability to come up with a simple and effective solution, there is clearly a need to have a method and apparatus for significantly reducing the warpage in semiconductor packages, and particularly, in TQFPs.